The expression "near-infra-red spectroscopy" is used to designate methods of measurements based upon the interaction between matter and electromagnetic radiation in the wavelength range from 700 to 2500 nm. The reason for using this expression is that it refers to the part of the infra-red wavelength range lying closest to the visual range of the spectrum (400 to 700 nm). In the literature, the expression "near-near-infra-red range" is used for electromagnetic radiation with wavelengths from 700 to 1200 nm.
Near-infra-red spectroscopy is used for determining components of various materials, e.g. in meat products. Meat consists substantially of water, protein and fat. Each type of chemical bond such as O--H, C--H, C.dbd.O, C--N, N--H, absorbs light at wavelengths characteristic for the molecule part concerned. The cause of the absorption is that two different atoms being bonded to each other function in the manner of an electric dipole taking energy from the electric and magnetic fields in the radiation, making the group of atoms concerned vibrate. Thus, a C.dbd.O bond in a triglyceride will absorb light at a wavelength, that is different from that absorbed by a C.dbd.O bond in a protein molecule. By measuring how much the light is attenuated by passing through a sample of meat at one of these characteristic wavelengths it is possible to determine the percentage of a component of the meat.
Measurements in the near-infra-red range may be carried out in two ways, either by passing light through the sample (near-infra-red transmission, NIT) or based on the reflection from the surface of the sample (near-infra-red reflection, NIR). In samples with a high water content, such as meat, NIT cannot be used when making measurements above 1300 nm, because the absorption by the water molecules is far too strong at longer wavelengths. With measurements based upon the reflection there is the disadvantage that they have to be carried out either on a free surface, which is not well-defined, or through a glass window. In the latter case, it cannot be avoided that fat on the comminuted meat adheres to the inside of the glass window, possibly causing erroneous measurement. Further, due to the small measuring volume, measurements based on reflection will not be as representative as NIT measurements.
Various analysis apparatus for examining materials by means of NIT spectroscopy are known. One of these apparatuses comprises a number of cups, in which a homogenized sample is placed. Then, the absorption of the sample is measured at a number of different wavelengths, and the content of components is computed on the basis of the absorption values having been found. The apparatus is extremely complicated to use. Thus, it is necessary to take a sample that is representative of the material to be examined, then the sample has to be homogenized, and finally the homogenized material has to be placed in the cups of the apparatus using great care. After this, the analysis may be carried out.